“Optical” or “photonic” switches allow selective transmission of electromagnetic signals, in particular the selective transmission of light. Photonic switches as used herein refers to any optical coupling or element having a source or input signal, an output or drain signal, and a third “gate” signal that affects the relationship between the source and gain signals. One such type of photonic switch is an optical transistor.
In a known embodiment, an optical switch can be formed using a photonic crystal. FIGS. 1A, 1B and 1C illustrate known one-dimensional photonic crystals. In general, such photonic crystals may be constructed by forming multiple, alternating layers of two materials having differing refractive indexes. If the thickness d1, d2 of each of the layers satisfies certain parameters (shown in FIGS. 1A and 1B), then the photonic crystal will display a “band gap” at certain wavelengths. In the case of one-dimensional (i.e. planar) photonic crystal as shown, the band gap appears for light directed normal to the plane of the device. That is, the photonic crystal will transmit light at wavelengths outside the band gap, but will reflect light of wavelengths falling within the band gap (for light generally orthogonal to the plane of the photonic crystal).
In FIG. 1A, for example, photonic crystal 10A is formed using alternating layers of a first material 11 having a refractive index n1, and a second material 12 having a refractive index n2. If the thicknesses d1 and d2 of the two layers are constructed to satisfy the equations d1=λc/4n1 and d2=λc/4n2, then photonic crystal 10A will display a band gap around wavelength λc. This exemplary band gap function is denoted by numeral 16 in FIG. 1C. As can be seen in the Figures, blue light, for example, having a wavelength λblue falling within band gap 16, may be reflected by photonic crystal 10A. In contrast, red light having a wavelength λred falling outside band gap 16 may be transmitted by photonic crystal 10A. These results are illustrated schematically in FIG. 1A. It should be noted that the red and blue colors and wavelengths specified herein are completely exemplary, and are utilized purely as a matter of convenience and clarity.
FIG. 1B illustrates a second photonic crystal 10B formed with layers of first material 11, having refractive index n1, and third material 13, having refractive index n3. In this case, assuming n3>n2, as designated in FIG. 1C, the band gap of photonic crystal 10B may be shifted, for example, to higher wavelengths, as shown by the reference number 17 of FIG. 1C. In contrast to the photonic crystal 10A, in this case (again by way of example) blue light having a wavelength λblue falls outside band gap 17, and therefore may be transmitted by photonic crystal 10B. Also in contrast to photonic crystal 10A, red light having a wavelength λred falls within band gap 17, and therefore may be reflected by photonic crystal 10A. These results are illustrated schematically in FIG. 1B.
An optical switch may be formed from a photonic crystal by using a non-linear optical material in place of second material 12 and third material 13. In a non-linear optical material, the refractive index changes non-linearly as a function of electric field strength and linearly with intensity, according to the function shown in FIG. 2C, where X(3) is a material-dependent function of wavelength. In this manner, the refractive index difference between the two materials forming the photonic crystal can be adjusted, causing a shift in the device's band gap.
FIGS. 2A, 2B and 2C illustrate a photonic crystal 20 formed from a first material 11 and a non-linear optical material 22, which has a refractive index that changes linearly with changing intensity and non-linearly with changing wavelength. As illustrated in FIG. 2A, when no gate signal is applied to photonic crystal 22, it obtains a band gap function as shown, for example, by reference number 26 of FIG. 2C. In contrast, when a gate signal such as λgreen is applied, as shown in FIG. 2B, the difference of the refractive indices of non-linear optical material 22 and first material 11 increases. This increased ratio results in a shift in the band gap, for example to the function denoted by reference number 27 of FIG. 2C. The materials, thicknesses, and wavelength employed are selected so that a selected wavelength of an input signal falls within the first band gap 26, but outside the band gap 27 (as shown with λblue), or vice versa (as shown with λred). Gate signal λgreen can therefore be selectively applied to selectively transmit or reflect input signal λblue or input signal λred, provided that λblue and λred are of such intensity as to not shift the band gap themselves.
It should be noted again that the colors and wavelengths specified herein are exemplary. In practice, given a selection of materials, a certain device may gate a red source signal with a blue gate signal, or vice versa. More generally, it may be possible to gate any particular color of light with another color, given a proper selection materials and design parameters. The exemplary colors used herein for clarity should not be viewed as limitations on the scope of the invention.
Current photonic switches constructed in this manner suffer a number of shortcomings. In some cases, the incidence of a gate signal and input signal each affect the refraction index of the non-linear optical material. Thus the signals must be carefully controlled, so that the input signal itself does not adversely affect the desired shift in the band gap. Current photonic switches may also suffer interference effects between the input and gate signals that carry through to the output signal, so that the gate signal contaminates the output signal. This may occur, for example, when the input signal and gate signal are co-axial, or have substantial components in the same direction. In addition, construction of photonic crystals as described is often a time-intensive or rigorous procedure.